Combustion engines may employ emission controls or systems that are configured to reduce the amount of nitrogen oxides (NOx), such as nitrogen dioxide, present in the engine's exhaust gas. One aspect of controlling such emissions may include the use of a NOx particulate filter (NPF) that has a Selective Catalytic Reduction (SCR) system and a particulate filter, such as a diesel particulate filter (DPF). The particulate filter is configured to remove particulate matter, such as soot, from the exhaust gas. The SCR typically uses a SCR catalyst, which, in some designs, may be coated on the particulate filter, and a reductant to convert NOx in the exhaust gas into nitrogen gas and water. Typically, the reductant is injected or dosed into the exhaust gas before the exhaust gas enters the NPF. The reductant may be a liquid or gas, such as, for example, ammonia (NH3), among others. At least a portion of the reductant that is injected into the exhaust stream is absorbed onto the SCR catalyst where, with the assistance of the catalyst, the reductant reacts with the NOx in the exhaust gas to form water vapor and nitrogen. In order for NOx to be converted into nitrogen and water vapor, the SCR catalyst may be required to store an adequate amount of reductant.
The amount of reductant that the SCR catalyst is able to store or absorb may decrease as the temperature of the exhaust gases that encounter or are around the SCR catalyst increase. Accordingly, a set-point may be established, and adjusted during vehicle operation, that indicates the reductant storage capacity of the SCR catalyst. Moreover, such a set-point may be established in an attempt to prevent excessive amounts of reductant from being present in the exhaust gas stream, such as excessive amounts due to a reduction in the SCR catalyst's reductant storage capacity and/or through reductant dosing levels. The presence of excess reductant in the exhaust gas due to the reduced storage capacity of the SCR catalyst may result in, or increase the probability of, reductant slipping through the after-treatment system and wasting the reductant.
During certain operating conditions, the temperature of the exhaust gas may be elevated relatively rapidly. For example, a relatively quick and significant increase in engine load may result in a relative quick elevation in exhaust gas temperatures. Yet, such rapid elevation in temperature(s) may not allow for the time necessary for consumption of the stored reductant, an associated adjustment in the quantity of reductant that is to be stored on the SCR catalyst and/or an adjustment to the quantity of reductant that is being injected into the exhaust gas stream. In such situations, the decrease in the reductant storage capacity of the SCR catalyst may result in the presence of excess reductant in the exhaust gas that may, at least potentially, slip out of the after-treatment system wasting the reductant.